The present invention relates to a heat pump having a plurality of compressors operated in parallel. More particularly, it relates to a heat pump in which the supply of oil to the compressors is improved.
A conventional heat pump of the type having a plurality of compressors connected in parallel is illustrated in FIG. 1. The illustrated apparatus is used as a heat pump for heating or cooling a building. A pair of compressors 1 and 2 are connected in parallel between a first heat exchanger 3, which in this case is an indoor heat exchanger, and a second heat exchanger 4, which in this case is an outdoor heat exchanger, via a 4-way valve 7 and piping 20. The two heat exchangers 3 and 4 are connected with one another via an expansion valve 5. An accumulator 6 is connected between the 4-way valve 7 and the intake side of one of the compressors so that all refrigerant returning to the compressors passes through the accumulator 6. In this manner, a closed loop is formed along which refrigerant can flow from the compressors 1 and 2 to the outdoor heat exchanger 4, through the expansion valve 5, to the indoor heat exchanger 3, through the accumulator 6, and back to the compressors 1 and 2 or in the reverse direction. The 4-way valve 7 enables either of the heat exchangers to be connected to the discharge side of the compressors while the other heat exchanger is connected to the intake side of the compressors via the accumulator 6 so that the apparatus can be operated in either a heating or cooling mode.
The bottom portions of both compressor are connected with one another by an oil equalizing pipe 15 through which oil can flow between the compressors when there is an imbalance in the amount of oil in the compressors. It also serves to prevent refrigerant from accumulating inside a stopped compressor, as well as to maintain the temperature of a stopped compressor at about the same level as a compressor which is running by passing a portion of high-temperature refrigerant from the compressor which is operating to the compressor which is stopped.
In such an apparatus, the compressors may be operated both at the same time or only one at a time, depending on the heating or cooling load.
During operation, lubricating oil for the compressors is continuously discharged from the compressors due to entrainment in the refrigerant, and the oil circulates through the heat exchangers and piping together with the refrigerant. When the piping connecting the compressors with the heat exchangers is extremely long, it takes a long time for the oil to circulate through the piping and return to the compressors. This can result in a shortage of lubricating oil developing in one or both of the compressors, producing jamming and damage to the compressors. This is particularly the case at start-up of the compressors, when foaming produces differences between the compressors in the amount of discharged oil and in the amount of returning oil.
Another problem with this conventional apparatus occurs when the compressors are stopped. Refrigerant and oil which remains in the piping at the time of stopping the compressors is free to return to the intake and discharge sides of the compressors due to gravity. The intake and discharge opening of the compressors can become filled with condensed refrigerant and lubricating oil, which can cause damage to the valves of the compressors when the apparatus is restarted.